JOURNAL OF RESEARCH ohhe National Bureau of Standards-A. Physics and Chemistry Vol. 80A, No.1, January- February 1976

Heat Capacities of Polyethylene IV. High Molecular Weight Linear Polyethylene

Shu-Sing Chang

Institute for Materials Research, National Bureau of Standards, Washington, D.C. 20234

(August 27, 1975)

A high mo lecu lar weight lin ear polyethyl ene samp le has been studied by ad iaba ti c c a lorimetry from 10 to 380 K. Two broad tem perature regions of unus ual spontaneous temperature drift have been observed. Th e phenomena occ urring around 240 K are simil ar to tha t observed in other poly· e thylene samples st. udied in t.hi s se ries , and a re pres umed to be caused by t.he re laxa tional processes in the amorphous phase. The weak exo th ermic behav ior occ urring around 160 K is pres umed to be ca used by the s tabi li zation of the que nched sa mple.

Ke y words : C lass transition; hea t capacity; hi gh molecu lar weight linear polye thylene; polyethylene; temperature drift ; thermodynam ic properties .

1. Introduction

The heat capacity be havior of a sample of high molec ular we ight linear polyethylene has bee n studied recently by Beatty and Karasz [1 , 2].' High molecular weight polyethylene is more difficult to crys tallize than lower molecular weight polyethylene. Increased amorphous conte nt s hould provide a stronger indica­tion of the exis te nce of a glass transition in partially crystalline linear polyethylene. A region of rather abrupt chan ge in the heat capacity differences be­tween that high molecular weight sample and a linear polyethylene sample studied by Dainton et aL [3] was noted. For the quenched high molecular weight sample, there was an 8 percent increase in the heat capacity occurring in the temperature region 130 to 190 K. For the annealed sample, there was about 5 percent in­crease. At higher temperatures a bend in the heat capacity curve was noted similar to Dainton's data. The region near 150 K was assigned as the glass tran­sition temperature of linear polyethylene. Spontaneous temperature drifts under adiabatic conditions, indica­tive of relaxational processes, were not noted in detaiL

A similar high molecular weight linear polyethylene sample from the same manufac turer was s tudied in this laboratory in the te mperature range from 10 to 380 K. De tailed s pontan eous temperature drifts were carefully observed on the sample under various thermal treatments. The results and conclusions from thi s

I Figures in brac ke ts ind ica te the lite rature references at the end of thi s paper.

51

s tudy are so mewhat different from the previously mentioned literature.

2. Experimental Section

2.1. Calorimetry

Heat capacity measure me nts on the high molec ular weight linear polyethylene sample were made with the same vacuum adiabatic calorimeter [4] that was used in the previous studi es [5, 6] on polyethylene samples derived from Standard Reference Materials 1475 and 1476.

2.2. Materials

A sample of high molecular weight polyethyle ne was furnished by Dr. R. J. Schaffhauser of the Plastics Division, Allied Chemical Corp.,2 bearing the designa­tion 260-100. This resin was at one time also designated as AC8X, as was Beatty's sample [2]. It was produced by Ziegler-type vapor phase polymerization. Its molecular weight is estimated by the manufacturer as 8-13 X 1()6. It has an ash content of 0.04 percent and contains no additives. The molecular weights are estimated in the order of 2.7-3 X 106 by both viscometry and light scattering measurements from the Polymer Characterization Section of the National Bureau of Standards. Whether the d.iscrepancy in the molecular weights is caused by the degradation of the polymer

1 Commercial mat erial s a re identi fied in thi s paper to adequately specify the experimental procedure. Such identifica tion does not impl y recomme ndation or e ndorsement by the Nalional Bureau of Standa rds.

o e

o 40

30

~ 20 .., .. Q,

u

10

o o

100

10

T, K 200

T,K 20

300

••

2

J 30

FIGURE 1. Heat capacity of high molecular weight linear polyethylene.

() Annealed at < 300 K, . annealed at 370 K, 0 quenched.

at high temperatures or by differences in the pro­cedures to estimate the molecular weight was not determined.

The sample was received in the form of a powder. Attempts to fuse the powder together under its own weight in vacuum at ISO °C were not successful due to the high viscosity of the melt. The heating produced

52

a sintered product. In order to avoid degradation of the high molecular weight material at high pressure and temperature, molding was not tried_

The material was pressed into pellets of about 1.27 cm in diameter and about I cm in height in a hydraulic press at room temperature_ 73.891 g (mass in vacuum) of the material was loaded in the sample container for

at 3.8 cm Hg (5.1 kPa) at room temperature was sealed in to improve thermal conduction within the s ample container.

3. Results

The results of the heat cap acity measurements are tabulated in table 1 and shown graphically in fi gure l. Table 1 is separated into various sec tions according to the thermal treatment the sample received. Within the section , the data are arranged in the order of in­creasing initial te mperature of a series of heat capacity measure ments. The series are numbered in chronologi­cal sequence throughout table 1 in order to facilita te the tracing of the thermal history of the sample .

TAB LE 1. Heat capacity data oj high molecular weight linear polyethylene

T, K

6.13 6.62 7.38 8.33 9.26

10.18 11.18 12.33 13.64 15.13

9.11 9.96

11.06 12.27 13.37 14.48 15.80 17.50 19.45 21.51 23.87 26.59 29.63 32.96 36.62 40.68

C p, J K - , mol - ' II Quenc hed

SERI ES X

0.0507 .0623 .0843 .1170 .1558 .1982 .2484 .3130 .3954 .5016

SE RIES IX

0.1500 .1875 .2445 .3106 .3773 .4536 .5549 .6963 .8758

1.082 1.335 1.649 2.015 2.437 2.916 3.453

T, K Cp, J K- 'mol- '

SERI ES lU

104.82 114.82 124.63 134.58 144.57 154.63 164.68 174.62 184.56 194.64

9.895 10.59 11.24 11.88 12.51 13.16 13.81 14.46 15.11 15.78

SERIES VIII

113.50 123.38 133.50 143.76 153.86 163.71 173.57 183.45 193.35 203.39

10.50 11.16 11.81 12.46 13.11 13.74 14.39 15.06 15.69 16.36

SERIES Xl SERIES IV

42.32 47.20 53.44 60.36 67 .54 74.97 82.91 91.40

100.88

3.663 4.296 5.085 5.906 6.690 7.436 8.171 8.882 9.609

205.32 215.33 225.29 235.29 245.31 255.21 265.11 274.99 284.83 294.73

16.51 17.21 17.92 18.69 19.55 20.44 21.38 22.37 23.41 24.55

53

T ABLE 1.

T, K

205.41 215.44 225.40 232.41 236.45 243.46 253.42 263.36 273.38 283.37 293.33

JJ 1.21 120.85 130.04 139.42 149.33 159.23 169.02 178.84 188.70 198.69

Heat capacity data oj high rnoLecular weight Linear poLyethylene - con tinued

C,), J K- ' mol - ' II T, K

Que nc hed

SERIES V

16.50 17. 19 17.92 18.45 18.77 19.40 20.29 21.23 22.21 23.26 24.35

303.29 313.08 322.98 332.98 342.96 353.02 363.28 370.79

Cp , J K - I mol - '

S ERIES XII

25. 70 26.96 28.27 29.80 31.43 32 .84 34.56 35.52

Slow·Cooled from 295 K

SERIES I

10.35 ]0.98 11.57 12. 16 12.79 13.41 14.02 14.64 15.26 15.91

201. 20 210.86 220.65 230.44 240.24 250.03 259.82 269.70 279.57 289.33

SERIES IJ

16.06 16.75 17.49 18.30 19. 14 20.02 20.95 21.92 22.92 23.96

Quenched and then annealed at 225 K, and 143 K

SER IES VI SERIES Vll

110.Q7 10.27 209.60 16.67 120.01 10.94 219.52 17.42 129.92 11.57 229.35 18.23 139.87 12.20 239.08 19.10 149.65 12.83 248.78 20.03 159.42 13.44 258.62 20.84 169.42 14.07 268.58 21.76 179.44 14.70 278.62 22.76 189.48 ]5.34 288.71 23.84 199.55 16.00

Annealed a t 370 K

SERIES XIII

283.27 292.99 302.85 312.75 322.67 332.59 342.49

23. 10 24.16 25.33 26.54 27.87 29.29 30.88

SERIES XI V

340.49 349.78 358.71 367.28

30.54 32.17 33.95 36.05

Slow·Cooled from 370 K

96.80 106.83 116.47 126.09 135.86 145.80 155.72 165.63

SERIES XV

9.29 10.03 10.69 11.31 11.96 12.55 13.17 13.79

SERIES XV ll

228.40 18.00 238.11 18.84 247.90 19.72 254.90 20.29 262.09 2J .01 272.14 21.97 282 .08 22.99 292.09 24.05

TABLE 1. Heat capacity data of high molecular weight linear polyethylene -continued

T,K Cp,J K- 'mol - ' II T,K Cp, JK - 'mol-'

Slow·Cooled from 370 K

SERIES XVI SERIES XVIII

172.39 14.21 297.52 24.63 182.23 14.83 307.32 25.90 192.21 15.45 317.06 27.14 202.23 16.10 326.91 28.46 212.18 16.79 336.84 29.93 222.06 17.52 346.75 31.57

356.51 33.45 366.15 35.72

Quenched samples were produced by admitting helium gas to the cryostat while the assembly was submerged in liquid nitrogen. A cooling rate in the order of 4-5 K min - 1 was achieved. Slow-cooling (rate annealing) rates of 0.5-1 K h - 1 were often used in conjunction with isothermal annealing (soak annealing) at a particular temperature for a period of days.

The temperature increment for a heat capacity determination may be inferred from the differences in the mean temperatures of the adjacent determinations within the series. Curvature corrections have been added to correct for the effect of the finite temperature increment of a determination. The precision of the measurement above 25 K is in the order of 0.05 per· cent. Below 25 K, the precision gradually changes to about 1 percent at 5 K. The accuracy over most of the temperature range of the measurement is comparable to the precision as indicate d by heat capacity measure· ments on a Calorimetry Conference standard sample of sapphire [4].

Smoothed heat capacity values representing those of a quenched (5 K min - 1 ) high molecular weight linear polyethylene are listed in table 2. Enthalpy and entropy increments referring to the zero point enthalpy and the residual entn?py of the sample are also listed. Since the partially crystalline sample is expected to have undetermined residual entropies at 0 K, Gibbs free energy is not given in table 2.

TABLE 2. Thermodynamic properties of high molecular weight Linear Polyethylene

(- CH2 -=14.027)

T,K Cp , J K- 'mol - ' H - Ho, J mol - ' S-So, J K- ' mol - '

5 0.037 0.047 0.012 10 .190 .527 .073 15 .492 2.175 .203 ~o .929 5.677 .401 25 1.463 11.63 .664 30 2.062 20.41 .983 35 2.702 32.31 1.349 40 3.360 47.47 1.752 45 4.016 65.91 2.186 50 4.656 87.60 2.642 60 5.862 140.3. 3.600 70 6.946 204.4 4.586

TABLE 2. Thermodynamic properties of high molecular weight

Linear Polyethylene -continued (- CH 2 -= 14.027)

T,K Cp , J K- 'mol - ' H-Ho,J moi - ' S-SQ, !~- ' mol - '

80 7.909 278.8 5.578 90 8.769 362.3 6.560

100 9.545 453.9 7.525 110 10.26 553.0 8.469 120 10.94 659.0 9.391 130 11.59 771.7 10.29 140 12.22 890.7 11.17 150 12.86 1016. 12.Q4 160 13.50 1148. 12.89 170 14.15 1286. 13.73 180 14.81 1431. 14.55 190 15.47 1582. 15.37 200 16.14 1740. 16.18 210 16.82 1905. 16.99 220 17.52 2077. 17.79 230 18.27 2256. 18.58 240 19.09 2443. 19.38 250 19.96 2638. 20.17 260 20.90 2842. 20.97 270 21.87 3056. 21.78 280 22.89 3280. 22.59 290 24.00 ' 3514. 23.42 300 25.20 3760. 24.25 310 26.51 4019. 25.10 320 27.90 4291. 25.96 330 29.38 4577. 26.84 340 30.91 4877 27.74 350 32.46 5194. 28.66 360 34.00 5526. 29.60 370 35.44 5877. 30.55 380 36.76 6248. 31.51

273.15 22.18 3125. 22.04

298.15 24.97 3714. 24.10

4. Discussion

The crystallinity of 45 percent for this powdery sam­ple was estimated from heat of fusion measurements using a dynamic scanning calorimeter. Heat of fusion of a . pressure crystallized linear polyethylene (96% crystallinity) [6] was used as the reference.

Although the high molecular weight sample has lower crystallinity than the SRM 1475 in the condition as received (71 % crystallinity) [5], it shows a somewhat lower heat capacity over the temperature range 90 to 310 K. Only at lower and higher temperatures does this sample exhibit the expected higher heat capacity. The heat capacity of this sample is very similar to that of a sample of SRM 1475 slowly cooled from the meit (88% crystallinity) [6] in the temperature range 100 to 220 K.

In a plot of . heat capacity differences, figure 2, between the heat capacity of this sample and that extrapolated for crystalline linear polyethylene [6], the changes in the 150 and 240 K regions are not as large as that for the 71 percent crystallinity sample. Beatty's data [2] are also included in figure 2. A rela­tively rapid heat capacity increase over that of the crystalline linear polyethylene occurs in the tempera-

54

ture region of 120 to 170 K. Above 200 K, the scattering of the data causes difficulties in the conclusion of either the existence or nonexiste nce of a subtle heat capacity change. DT A or DSC studies on some other high molecular weight polyethylene samples sug· gested the occurrence of a step change in the heat capacity around 150 K [7 , 8] and indicated a s Uf·

prisingly constant heat capacity difference between

5

4

3 "7 -0

E "7 ~ ....,

CL.

u <J

2

.. ..

6

the high molecular weight sample and a high crystal· linity sample [8].

The lack of a strong glass·like feature in the heat capacity behavior of this high molecular weight rna· teri al is in parallel to a recent thermal expansivity study on the same materials. Low temperature thermal expansions of polye thylene samples have been studied on strips of compression molded thin films [9]. The

.. ...... ..

.. .. 0 .... 0

0

0

o

.. .. ..

o

.. 0 o 0

.. 0

0

DO

.. .. ..

..

.. o

o

o o

o o

o

8 0 .. 0 ~ 0

i/o

o 100 200 T, K

1% I 1%

300

F IGURE 2. Heat capacity differences of high molecular weight linear polvethylene.

Th is research : ( )and () a nnea le d al < 300 K:. annea led at 370 K; O . <S> and e que nc hed. Beatty 12 J:6 run I; T rUll 2; 0 run 3.

55

samples included the two Standard Reference Ma­terials 1475 and 1476, used in a previous calorimetric study [5], and the high molecular weight sample used in the present calorimetric work. In all three samples, double peaks in the derivatives of the thermal expansion coefficients at about 110 and 150 K were observed. These peaks are stronger in linear poly­ethylene than in branched polyethylene. Weaker peaks were observed in the temperature range 180 to 200 K. Above 200 K, there were strong peaks in the branched polyethylene, a weak one in the high

100 200

decay of the temperature of the thermometer until a thermal equilibrium is reached between the sample and the sample container assembly. The decay con· stant depends on the construction of the sample container, the thermal conductivity of the sample and the amount of helium gas sealed in the sample con­tainer. The decay constant is about 50 s for this sample container [12] at high temperatures and less at lower temperatures. Therefore up to 10 to 15 minutes may be required for the sample to reach a temperature distribution which is uniform to within 10- 4 K

300 400 T. K

FIGURE 3. Spontaneous adiabatic temperature drift of high molecular weight linear polyethylene.

Open circles and squares (quenched), <D Ser. III and IV. e V. <SI VlII, 0 XII, <> XIII, 0 XIV. 0 before Ser. XV and ® before Ser. IX , drift observations only. Filled or half-fi lled circ ules (annealed): () I and II , . XV-XVII.

molecular weight sample and only a monotonic In­

crease in the ordinary linear polyethylene. Although the appearance of a discontinuity in the

heat capacity behavior is perhaps a necessary condition for a glass transition, it is not a sufficient condition, A broad distribution in the relaxation times may cause the glass transformation to occur over a wide tempera­ture range, thus making a weak heat capacity disconti­nuity in partially crystalline material even more difficult to assess. Since the glass transition is kinetic in nature, relaxational phenomena should also be observed in the glass transition region. One highly sensitive method to detect the thermal relaxation is the method of observ­ing the spontaneous temperature changes of the sample under adiabatic conditions [10]. The sign and the magni­tude of the temperature drift as a function of time and temperature may be correlated with different thermal treatments and histories. Both the discontinuity in the heat capacity behavior and the adiabatic temp~rature drift are commonly observed in glass transition regions of various materials and also in glass-like transitions in crystals [11].

The drift observations are shown in figure 3. These are the drift rates observed at 20 minutes after the heater energy has been turned off. Because of the geometric arrangement of various components in the sample container [4], the temperature of the ther· mometer and the heater is slightly higher than that of the sample during the heating period. Therefore, when the energy is turned off, there is an exponential

The drift behavior in the 240 K region for the high molecular weight material occurs in similar tempera· ture range and magnitude as that observed for the as­received SRM 1475 [6, 10] and is presumed to be caused by the relaxational behavior in the amorphous phase. For the quenched sample, i. e. , cooling rate is greater than the rate of heat capacity determination of 5-10 K h - 1, the drift rate is generally positive (exothermic) with a broad peak around 230 K There is also a weaker peak at around 160 K When the sample was heated to 370 K for the first time, large exothermic behavior showed up above 330 K, indicating the onset of further crystallization processes. For the sample either slow­cooled continuously to 100 K or annealed at 225 and 140 K, there is an endothermic peak only around 245 K. No endothermic behavior was observed at tempera­tures below 210 K for the sample either slow cooled or annealed. Thus the weaker exothermic peak occurring around 160 K for quenched samples is presumed due to some stabilization processes such as the relief of strain.

In the broad temperature region near 240 K, the drift behavior as observed in figure 3 and in other linear polyethylene samples [6, 10] seems to indicate the exist­ence of double peaks. Special thermal treatments were performed in attem,pts to resolve these peaks. Figure 4 shows the results of the drift observations for two combinations of quenching, annealing and slow cooling treatments. The sample was first annealed at 250 K for 1 day and then quenched from 250 K Upon

56

0.001

·0001

100 400 T. K

FIGURE 4. Spontaneous adiabatic temperature drift of high molecu lar weight linear polyethylene.

® Annealed at 250 K and then quenched from 250 K (before Ser. IX). f) Quenched from 360 K and then annealed al 225 K and 143 K (Scr. VI and V II ).

heating the sample, it showed exothermic be havior, similar to that of a quenched sample, at temperatures below 250 K. Above 250 K, e ndothermic e ffects simi· lar to an annealed sample were seen.

The sample was quickly cooled (5 K min - 1) to 225 K and held there for 4 days, until the drift decreased from an initial value of greater than 1 mK min - 1 to less than 0.05 mK min - I. It was then slowly cooled (2.5 K h - I )

to 143 K and held there for 2 days. An e ndothermic peak was observed at about 245 K. Above 245 K, a small exothermic peak appeared near 270 K. Ap· parently annealing at 225 K did not relax the configura· tions associated with the higher temperature peak.

The two regions around 240 and 270 K may be caused by different types of amorphous relaxation mechanisms, such as from the loose ends of the poly· mer molecules or from the molecular segments with both ends confined in crystalline regions. However, the mechanisms responsible for the relaxational behavior observed here have not been determined.

The author wishes to thank R. G. Christensen for the molecular weight determinations and C. H. Pearson for the assistance in calorimetric measurements.

5. References

[1] Beatty, C L. , and Karasz, F. E., Bull. Am. Phys. Soc. Ser. II, 16,1391 (1971).

[2] Beatty , C L., Ph. D. thes is, University of Massachusetts (1972). [3] Dainton, F. S., Evans, D. M., Hoare, F. E. , and Melia, T. P. ,

Polymer 3, 277 (1962). [4] Ste rrett, K. F., Blackburn, D. H., Bes tul, A. B. , Chang, S. S.,

and Horman, J. A., J. Res. Nat. Bur. Stand. (U.S.), 69C (Engr. and lnstr.) No.1, 19-26 (Jan.-Mar. 1965).

[5] Chang, S. S .. and Bes tul , A. B. , J. Res. Nat. Bur. Stand. (U.S.) 77 A (phys. and Che rn .), No.4, 395-405 (J uly-Aug. 1973).

[6] Chang, S. S'-; J. Res. Nat. Bur. Stand. (U.S.), 78A (phys. and Chern) , 0. 3,387-400 (May-June 1974)

[7] Stehling, F. C, and Mandelkern , L. , Macromol. 3, 242 (1969). [8) IIJers, K. H., KoU. Z 250,426 (1972); 252,1 (1974). [9] Lee, S., and Simha, R., Macromol. 7,909 (1974).

[10] Chang, S. S., J. Polymer Sci. Sym. 43,43 (1973). [11] Suga, H., and Seki, S., .1 . Non·cryst. Solids 16,171 (1974). [12] Chang, S. S., Rev. Sci. Tnstr. 40, 822 (1969).

(paper 80AI-881)

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